Under a skilled rider, a Tiger 800XC can be ridden on technical ATV trails like a dirtbike, and it can also fit a 21" front DOT knob with a capable rear DS tire like a K60. The 800XC is more than capable of blazing slab all day, with a full load and a passenger, at over 100MPH. Why somebody who can't ride gravel without a second thought needs a 550lb+ 1200cc bike for solo adv-riding, in a country with sub-90MPH speed limits, is beyond me.

With that said, airing down can improve the feel on unpaved surfaces. So can using appropriate tires. So can attending a course of instruction that is focused on riding bigger bikes off pavement.

Why a large bike? Because riding 500 miles in a single day on the highway in a smaller bike is literally a pain in the butt. I can take a couple of hours of uncertainty on gravel for many hours of comfort on road

I ride a Husky TE610, it's about 300 pounds, I'm 230. I can tell you that with Motoz Tractionator tires and rim locks, it has ridiculously awesome grip on twisty smooth gravel roads at 8psi in the front, 12psi rear. I wouldn't try that for more than a few miles, and I wouldn't try it with other tires, and I wouldn't try it with a heavier bike, or without rim locks. So what good is my advice? Very little... except to expose that the "right" tire pressure has a very wide range and is dependent on lots of factors.

And having a CO2 cartridge to air up your tires gives you a lot of options.

Why a large bike? Because riding 500 miles in a single day on the highway in a smaller bike is literally a pain in the butt. I can take a couple of hours of uncertainty on gravel for many hours of comfort on road

It's not like I suggested a 250cc 2-stroke MX ring-dinger with a rock-hard seat as narrow as a 2x4...on edge. You can even upgrade the seat on most bikes these days too. Try some lighter bikes like the Tiger 800XC, F800GS, CB500X, G650GS, KTM 950 or 990, or even a DL650. You might be surprised at the long-distance comfort on some of the smaller and lighter adv bikes. Choosing a 600lb+ behemoth GS over something like a Tiger 800XC because of solo 500-mile comfort in a country with speed limits seems funny to me. There just isn't that much difference in plushiness, and the lighter bike may even fit you better and/or run smoother down the highway with better wind management. Smoothness improvement has actually been reported by riders that downsized from a DL1000 to a DL650.

I've ridden a sub-400lb thumper dualsport 1100 miles in a day on a stock seat. I've also done cross-continent slab trips on a 440cc twin and an 850cc I4. It's really no big deal, and an aftermarket seat can make it even comfier. The 850 was even more comfortable for long days than the GL1200 we also had in the garage at the time. There's no legitimate reason why a smooth and peppy 450-500lb multi-cylinder with appropriate ergonomics and wind management can't burn 500+ miles/day down the average interstate in comfort.

My guess is that ego is involved. Let go of your insecurities and be free to try new things. You might be pleasantly surprised.